The storage and transportation of a wide variety of goods is greatly facilitated by the use of pallets. Pallets allow the storage and movement of different items by a common material handling system employing forklift trucks. In the early years of pallet usage, most pallets were constructed of hardwoods because of its low cost, ready availability and high compressive strength.
Wood pallets are still widely used in the industry. However, wood pallets are subject to splintering, moisture absorption, and the steel fasteners which hold wooden pallets together will rust if exposed to water. In general, wooden pallets have a limited use life. Plastic pallets are advantageously used where cleanliness, repeated usage or special attachment needs are presented.
All general purpose pallets share several basic structural properties. They have a generally flat upper deck for supporting boxes, canisters or crates, and they have two or more openings for the admittance of fork lift tines. The tine openings may be formed either between a pallet top deck and a pallet bottom deck, or the pallet may have only a single deck with an array of legs which support the deck above a support surface to allow entrance of fork lift tines beneath the deck.
Many manufacturing processes have been adapted to production of plastic pallets: injection molding, cellular foam, blow molding, and rotomolding. However, the large size of pallets, often four feet long or greater, makes the thermoforming process particularly well suited to the production of pallets.
One successful approach to a plastic pallet, such as disclosed in U.S. Pat. No. 4,428,306 to Dresen et al. is a pallet produced in a twin-sheet thermoforming process in which the upper sheet is fused to the lower sheet in the walls of downwardly protruding cup-like feet. Another approach to plastic pallets, such as disclosed in U.S. Pat. Nos. 5,197,396; 5,329,862; 5,351,629; and 5,413,052 to Breezer et al. utilizes twin-sheet thermoformed top and bottom decks which are connected by separately molded plastic leg posts. Such pallets can be adapted for high loads by positioning tubular steel reinforcements between the plastic sheets of the upper deck, the bottom deck, or both.
In the thermoforming process a sheet of thermoplastic material is heated until it becomes soft and moldable, but not fluid. The heated sheet is held against a mold, whereupon a vacuum is drawn between the mold and the plastic sheet, drawing the sheet down onto the mold, and causing the thermoplastic sheet to conform to the mold's surface. In twin-sheet thermoforming both an upper sheet and a lower sheet are heated and molded simultaneously in two separate molds. The heated sheets are then pressed together within the molds. The effect is to create an article which may have enclosed volumes, and regions of plastic of desired thicknesses.
Material handling products such as pallets are highly engineered products in which physical performance is weighed against cost. Because the material cost of the plastic resin which goes into a pallet is a substantial portion of the pallet's total cost, there is a great need to produce a pallet which has high performance and stiffness capacity with low resin weight.
Providing a pallet with two decks advantageously contributes to overall pallet stiffness. Nonetheless, multiple parts in a pallet contributes to increased cost due to separate molding operations, required fasteners, and labor required for assembly. Furthermore, the advantage of two decks toward unit stiffness is lessened when the parts connecting the decks are subject to lateral motion with respect to one another. What is needed is a double deck plastic pallet which is economically assembled from a minimum of parts, and yet which offers good structural performance.